In television systems, it is known to reduce one or both of the contrast and brightness of a reproduced image in order to inhibit "white-spot blooming" due to excessive electron beam currents of the cathode ray tube (CRT) displat device, as well as to inhibit CRT drive and phosphor amplifier saturation which tend to limit the "slew" rate of the electron beam. This may be accomplished by directly sensing the beam current and, in response, generating a control signal for the contrast and/or brightness control sections of the system. The control signal may also be generated by detecting a characteristic of a video signal coupled to the cathode ray tube. For example, U.S. Pat. No. 4,599,643, entitled "Apparatus Responsive To Plural Color Video Signals For Amplitude Limiting The Video Signals To Assist Beam Current Limiting", issued to W. E. Harlan, discloses combining the three color signals coupled to the cathode ray tube and detecting the average of the white-going peaks of resulting signal above a predetermined threshold to generate a contrast control signal.
In U.S. Pat. No. 5,003,394 entitled DYNAMIC VIDEO SYSTEM INCLUDING AUTOMATIC CONTRAST AND "WHITE STRETCH" PROCESSING SECTIONS" which issued to W. A. Lagoni Mar. 26, 1991, it is recognized that, while it is desirable to prevent spot blooming, for example, by automatically controlling the contrast of the reproduced image, such contrast reduction may reduce the contrast and subjective brightness of the reproduced image. More specifically, Lagoni recognized that while it is desirable to provide automatic contrast control apparatus to reduce the amplitude of the luminance signal when the reproduced image includes white-going peaks exceeding a predetermined level corresponding, e.g., to characters, mid-range luminance amplitudes will also be reduced. This results in a contrast and subjective brightness reduction.
To overcome this problem, Lagoni proposed a system in which a non-linear amplitude control section (hereinafter a "white stretch" processor) is coupled in cascade with the contrast control unit to dynamically emphasize mid-range amplitude luminance levels relative to high amplitude luminance levels as a function of the average value of the luminance signal processed by the contrast control apparatus. For images containing excessive white-going peaks but a low level average luminance component (hereinafter, a low "average picture level" or "APL"), the effect is to decrease the amplitude of the white-going peaks while simultaneously increasing the amplitude of mid-range luminance levels. In this way, "white spot blooming" (as well as CRT phosphor and driver saturation) can be minimized while providing subjectively sharp, bright images.